1. Field of the Invention
The present invention relates generally to telecommunications systems and methods for providing speech quality to mobile stations within a cellular network, and specifically to assigning channels to mobile stations based upon uplink interference levels and channel quality.
2. Background and Objects of the Present Invention
Mobile communications, especially cellular radio, is one of the fastest growing and most demanding telecommunications applications ever. Today it accommodates a large and continuously increasing percentage of all new telephone subscriptions around the world with the increasing service requirements. Cellular networks have evolved into two different networks within Time Division Multiple Access (TDMA) technology. The European cellular network uses the Global System for Mobile Communication (GSM) standard as the digital cellular system. In the United States, cellular networks have traditionally been primarily analog, but recent advances have been incorporating digital systems within the analog networks. One such North American cellular network is the D-AMPS network, which is described hereinbelow.
With reference now to FIG. 1 of the drawings, there is illustrated a D-AMPS Public Land Mobile Network (PLMN), such as cellular network 10, which in turn is composed of a plurality of areas 12, each with a Mobile Services Center (MSC) 14 and an integrated Visitor Location Register (VLR) 16 therein. The MSC/VLR areas 12, in turn, include a plurality of Location Areas (LA) 18, which are defined as that part of a given MSC/VLR area 12 in which a mobile station (MS) 20 may move freely without having to send update location information to the MSC/VLR area 12 that controls the LA 18.
Each Location Area 12 is divided into a number of cells 22. Mobile Station (MS) 20 is the physical equipment, e.g., a car phone or other portable terminal, such as a laptop, used by mobile subscribers to communicate with the cellular network 10, each other, and users outside the subscribed network, both wireline and wireless. The MSC 14 is in communication with a Base Station (BS) 24. The BS 24 is the physical equipment, illustrated for simplicity as a radio tower, that provides radio coverage to the geographical part of the cell 22 for which it is responsible.
With further reference to FIG. 1, the PLMN Service Area or cellular network 10 includes a Home Location Register (HLR) 26, which is a database maintaining all subscriber information, e.g., user profiles, current location information, and other administrative information. The HLR 26 may be co-located with a given MSC 14, integrated with the MSC 14, or alternatively can service multiple MSCs 14, the latter of which is illustrated in FIG. 1.
The VLR 16 is a database containing information about all of the Mobile Stations 20 currently located within the MSC/VLR area 12. If a MS 20 roams into a new MSC/VLR area 12, the VLR 16 connected to that MSC 14 will request data about that Mobile Station 20 from the HLR database 26 (simultaneously informing the HLR 26 about the current location of the MS 20). Accordingly, if the user of the MS 20 then wants to make a call, the local VLR 16 will have the requisite identification information without having to reinterrogate the HLR 26. In the aforedescribed manner, the VLR and HLR databases 16 and 26, respectively, contain various subscriber information associated with a given MS 20.
The radio interface between the BS 24 and the MS 20 can utilize, for example, Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA) to transmit information between the BS 24 and the MS 20. In TDMA, as shown in FIG. 1 of the drawings, one TDMA frame 24 is assigned per carrier frequency. Each frame 24 consists of six timeslots or physical channels 35. Depending upon the kind of information sent, different types of logical channels can be mapped onto the physical channels 35. For example, speech is sent on the logical channel, "Traffic Channel" (TCH) 37, and signaling information is sent on the logical channel, "Control Channel" (CCH) 38.
Currently, speech and data are transmitted from the BS 24 to the MS 20 on a downlink channel 30 and from the MS 20 to the BS 24 on an uplink channel 32. Interference on either the downlink channel 30 or uplink channel 32 can significantly reduce the quality of the signal transmitted on these channels. Two types of interference of interest are co-channel interference and adjacent channel interference. Co-channel interference is the interference caused by the usage of the same frequency within two different clusters (not shown) of cells 22. Adjacent channel interference is caused by the usage of adjacent frequencies between adjacent cells 22 within the same cluster or within two different clusters.
In analog systems, the carrier-to-interference (co-channel or adjacent-channel) (C/I) ratio is one of the most important radio network performance criteria in evaluating an analog cellular network 10, such as the AMPS network. In order to reduce interference within the cellular network 10, both co-channel and adjacent channel interference must be minimized. Therefore, by increasing the C/I ratio, e.g., by reducing the interference with respect to the carrier (level) of the desired signal, the co-channel or adjacent channel interference can be reduced and the signal quality received by MSs 20 within the cell 22 can be improved.
The speech quality in digital cellular systems 10, such as the Global System for Mobile (GSM) Communication network or the D-AMPS network, is measured via quantities such as frame erasure, which is the percentage of TDMA frames that cannot be perceived, and the bit error rate (BER), which is an estimate of the number of coded bits in error. In order to measure the BER, the encoded bits that are transmitted in each burst or frame of data across the downlink channel 30 or uplink channel 32 are received by a receiver (not shown) and decoded, using, for example, a convolutional decoding algorithm. The algorithm also estimates how many errors were induced by the channel. This estimate of the BER can be referred to as the raw BER. It should be understood that the number of errors estimated by the convolutional decoder is just an estimate of the actual BER. However, this estimate can be considered reliable to a certain degree, and since convolutional codes are usually the most efficient coding mechanisms employed, the BER can be considered as the best estimate of the deterioration in speech quality for digital cellular networks 10.
In order to ensure adequate speech quality for MS's 20, the assignment of a traffic channel 37 to an MS 20 involved in a call connection has traditionally been based upon the C/I or BER uplink measurements. Many different approaches to channel assignment have been proposed to date. For example, two commonly used channel assignment methods include the traditional fixed channel assignment (FCA) strategy, and the distributed minimum interference scheme. In the minimum interference scheme, the MS 20 is assigned the traffic channel 37 of the nearest BS 24 with the minimum uplink 32 interference.
Another type of channel assignment method is the multi-channel assignment (MCA) algorithm. In this approach, various C/I constraints are guaranteed to various subscriber services. As these services require different BER performance levels, different C/I values are used to meet these requirements. The cell 22 is typically divided into concentric zones and the C/I performance is traded-off according to the subscriber service requirements of each user.
A further type of channel assignment method is discussed in U.S. Pat. No. 4,794,635 to Hess. This method includes determining the channel 37 and sector activity and establishing a minimum quality factor for each active channel in order to establish an eligible channel 37 to which the requesting mobile subscriber will be assigned. The minimum quality factors attributable to such subscribers for each channel 37 are compared with respect to every other channel 37. The requesting mobile subscriber is assigned to a particular one of the possible eligible channels 37 which has a maximum of subscribers having the minimum quality factor.
In addition, in U.S. Pat. No. 5,212,831 to Chuang et al., a procedure is proposed that consists of signal strength measurements and an algorithm which selects the frequency with minimum interference from other ports. The frequency channel 37 with the lowest received power is tentatively assigned for downlink transmission by that port. All the ports repeat this procedure either independently or asynchronously or with a schedule.
The algorithms proposed in U.S. Pat. Nos. 4,794,635 and 5,212,831 disallow selection of channels 37 that have been disturbed by interference. As a first step, the C/I is calculated on the selected channel 37. The C/I is compared to the disturbed channel criteria to determine if the channel 37 is considered to be disturbed. A new channel 37 is re-selected if the channel 37 is considered disturbed.
However, in all of the above channel assignment methods, traffic channels 37 are either disallowed if they are considered disturbed or only chosen if they possess the minimum interference levels. In many cases, channels 37 that are considered disturbed may provide adequate speech quality for some MS's 20. In addition, if the channel 37 with the minimum C/I is always assigned, especially if there is congestion in the cell 22, one MS 20 may be assigned a channel 37 with a high C/I ratio that does not provide adequate speech quality to that MS 20, while another MS 20, which had previously been assigned a channel 37 with a lower C/I ratio, could have been assigned to the channel 37 with the higher C/I ratio and still experienced adequate speech quality. Thus, none of the above methods efficiently or effectively assign channels 37 to MS's 20 based upon the channel quality experienced by each individual MS 20.
It is, therefore, an object of the present invention to assign a channel to a mobile subscriber based upon both the interference level of the channel and the individual channel quality experienced by the mobile subscriber.